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This question has been hanging around in my head for a while; all the way from back in 2012, when we replaced our core network switch.

This switch was actually a stacked pair of old 3Com 4900sx, and it was responsible for layer 3 routing between VLANs in our network. We're just a two-man IT shop, where a lot of our internal expertise is more in workstation, server, and application support, with only basic network engineering, so we engaged a consulting firm for this project. The consultant's task was to look at how our network was configured, and what growth we were having, to make sure the new switch was set up well (not just matching what we had, but also looking to the future), as well as help with the cut-over to minimize downtime.

To make a long story short (too late), there were two noticeable configuration differences from the old to the new. The first difference is a much shorter ARP timeout. This makes sense to me; the rise of Wi-Fi means a device might move from one place to another and end up on a different port, invalidating cached ARP information. The second difference is the old switch used OSPF areas, but the new switch does not do any OSPF configuration at all. This part I do not understand. It's not so much that it doesn't make sense, as that I lack a frame of reference for knowing when or why one would want to use OSPF at all.

And so there is my question: when does it make sense to add OSPF to a network, or why would you choose OSPF over another option? I've been through several resources on this in the past, but never quite had this solved to my satisfaction.

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Routing protocols, such as OSPF, are to share routes between routers. If your routing is done on a single router, or a pair of routers connected to the same networks (your layer-3 switches), it doesn't make any sense to use CPU cycles for the routing protocol, since routers inherently know about directly connected networks. Both your layer-3 switches already have all the routes that the other layer-3 switch has.

OSPF is one choice for a routing protocol, but it is an industry standard, and just about every business-grade router supports it. It is also very well understood by most network engineers, and fairly simple to configure.

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  • I'm on a campus with 19 buildings. Some vlans may be used anywhere on campus, but a few are primarily used in specific buildings. The switches in several buildings are layer-3 capable, but not using that feature. Does this mean I might be able to improve network performance by allowing switches in some of the busier buildings to do routing for the localized vlans, and using OSPF to move routes around? I could see where, done right, this might allow some packets to avoid needing to go to the core switch.
    – Joel Coehoorn
    May 11, 2016 at 17:42
  • Only if you have routers with networks which the other routers do not have. If your layer-3 switches in the other building have the same networks as the layer-3 switches doing the routing, it doesn't make any sense to enable routing on them. You could, of course, use static routes, but that doesn't scale. You use a routing protocol for routers to tell each other about routes which the the other routers do not have.
    – Ron Maupin ♦
    May 11, 2016 at 17:45
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    Another way to think of it: routing protocols are used by routers to exchange routing information with other routers. Since you have only one router, there's no one else to talk to.
    – Ron Trunk
    May 11, 2016 at 17:53
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    Perhaps you could post your network diagram. Then we could offer better advice. You should make that a new question, though.
    – Ron Trunk
    May 11, 2016 at 17:54
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    I think I figured out why we made the change. The old network used OSPF to cut down on traffic that needed to move over the stacking link between the two 4900sx units, as well as between the 4900sx and the old gigabit copper layer-2 switch to which our servers were connected. Our new switch combines all of this into the same device, so that traffic simply moves through the switch fabric, and OSPF would no longer help.
    – Joel Coehoorn
    May 13, 2016 at 17:13
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This link has a good intro on ospf and design of campus architectures.

http://docstore.mik.ua/cisco/pdf/routing/OSPF Routing Protocol(1).pdf

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  • Could you please edit your post to provide more details from the link? To avoid problems with link rot, the community prefers that you quote important content and provide the link as reference whenever possible.
    – YLearn
    May 13, 2016 at 1:57
  • Link rot is a problem, but this link was helpful, and it would be difficult to summarize the content, as the article is long and detailed.
    – Joel Coehoorn
    May 13, 2016 at 17:11
  • @JoelCoehoorn - The link has been updated. ospf has been the internal routing protocol of choice for many campus administrators for many years as it isn't proprietary (thus multi vendors compatible), easy to setup and optimise. There are others such as IS-IS (many ISPs) and eigrp (fast but Cisco proprietary). But as in all campus designs regardless of protocols used, a good scalable design and securing who you advertise your routing protocol to; is mandatory. Networkengineering.stackexchage is a good site to ask for more questions if you require further assistance with your campus design :-)
    – user4565
    May 13, 2016 at 20:51
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You will see dynamic (automatic/automated) routing protocols like OSPF used in multi-site networks. with OSPF, if you add a subnet (new VLAN) at remote site-x, the routing tables of the other routers in the network can get updated automatically. Without OSPF, you might have to add that new route in every other site's router.

EIGRP is widely deployed in all-Cisco networks but a big advantage of OSPF is that it is standards based. So customer can integrate other vendors routers in the future.

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Read this article, superbly explained. http://learnwow.blogspot.in/2017/10/ospf-in-networking-top-standout-characteristics.html?m=1

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